In printer systems wherein an intensity-modulated beam of radiation (e.g., a laser beam) repeatedly scans a moving recording element to record image information, pixel-by-pixel, the velocity at which the recording element moves must be extremely uniform to produce high quality images. This also applies where the image information is being applied to a moving recording element line-by-line, such as with a linear array of light emitting diodes, or page-by-page as with an optical copier. If the velocity at which the recording element moves varies while the rate at which the image information is conveyed remains the same, there will be either a crowding together or spreading apart of lines of image information. This artifact, known as banding, causes a degradation in image quality.
In commonly assigned U.S. Pat. No. 5,140,370, issued Aug. 18, 1992 in the name of Kevin M. Johnson, entitled POSITION CONTROL FOR TRANSFER DRUM IN ELECTROSTATOGRAPHIC PRINTER/COPIER, there is disclosed an electrophotographic apparatus in which a scanning laser beam imagewise discharges an electrostatic charge on the surface of a photoconductive drum, leaving an electrostatic latent image. The electrostatic latent image is developed with colored toner particles from one of several development stations to create a transferable color toner image on the outer surface of the photoconductive drum. The toner image is transferred to an image-receiver sheet at a nip formed between the photoconductive drum and a transfer drum. The transfer drum is internally heated and its outer surface is urged into contact with the photoconductive drum surface at a relatively high force (e.g., 300-500 pounds).
When the leading edge of the image-receiver sheet enters the nip, it suddenly retards the rotation of the photoconductive drum, causing a sudden increase in the torque (force) required to rotate the drum at constant speed. Similarly, when the trailing edge of the image-receiver sheet leaves the nip, the drum torque is suddenly decreased, causing a temporary increase in drum speed. If the laser is recording image information at the time of these "torque spikes", variations in line spacing will occur, causing the above-described banding artifact to appear.
The above-mentioned velocity fluctuations have a particularly adverse impact on the quality of prints produced by multicolor printer systems. In the Johnson apparatus, several color separation toner images are superimposed on each other on the image-receiver sheet to form a multicolor image. If the lines of image information are not uniformly spaced apart for each color separation image, color misregistration will result in the composite image.